Stresses and strains that are created in a metallic material, for example, during machining are one of the important design factors that determine the reliability of not only the components manufactured from the material but also the final products assembled from the components. In the field of semiconductor apparatuses, device sizes are becoming increasingly smaller, and the defective current leak property due to changes in electron states or the presence of micro defects at the interface caused by residual stress or concentrated strain is becoming a major problem. As this trend is expected to continue in a more pronounced manner, there is a need for higher resolution, higher sensitivity, and faster means of measuring stress (strain measuring means). Measuring of stress (strain), which can be carried out based on a distribution of stress (strain) inside a region as well as via a pinpoint precise measurement of a specific point, can provide a great amount of important information.
Various methods of measuring stress/strain have been proposed for required resolution. The simplest method is by the use of a strain gauge. Strain gauges can be roughly divided into mechanical and electric resistance-wire types. The mechanical strain gauge measures by converting the amount of strain into a physical quantity such as the twisting of an indicating needle. The electric resistance-wire type strain gauge is a converter element taking advantage of the property of a metallic wire whose resistance increases as it is extended. The surface acoustic wave method measures strain or stress by taking advantage of the fact that the speed of a surface acoustic wave that is created when an ultrasonic wave is impinged on a material surface differs depending on the magnitude of the strain or stress. The micro-Raman method is based on the principle that the peak position of the energy of scattered light emitted by a material upon laser light irradiation varies depending on the stress/strain.
According to a convergent-beam electron diffraction (CBED), a sample is irradiated with an electron beam converged to a small size, and the stress/strain in a specific direction is measured based on the changes in the diffraction pattern of the transmitted electron beam. The CBED method is described in JP Patent Publication (Kokai) No. 2000-65762, or Journal of Applied Physics, Vol. 32 (1993), p. 211, for example. The Fourier transform method utilizes the fact that the lattice interval of a crystal lattice image recorded on a high-resolution photograph taken by a transmission electron microscope (TEM) varies depending on the stress/strain. By cutting out a portion of the high-resolution photograph and subjecting it to fast Fourier transform (FFT), the lattice interval can be precisely determined. The Fourier transform method is described in JP Patent Publication (Kokai) No. 2000-65762, for example. The micro (nano) diffraction method utilizes the fact that the position of diffraction spots in an electron beam diffraction pattern obtained by TEM changes depending on the stress/strain of the sample. The method is called a micro diffraction method when the spot size of the irradiated electron beam is on the micrometer order and a nanodiffraction method when the spot size is on the nanometer order.
Examples of the means of observing a micro region of a sample include TEM, a scanning electron microscope (SEM), and a scanning transmission electron microscope (STEM). Electron beam scanning techniques are commonly employed in SEM and STEM. Both of them employ an irradiation lens system to focus an electron beam, which is then two-dimensionally scanned using a deflection coil. The signal intensity distributions of secondary electrons, reflected electrons, scattered electrons, and transmitted electrons that are produced in synchronism with the scan are displayed as images. The techniques for displaying the multiple items of information obtained from these signals simultaneously are also known. In recent years, techniques have also become known for analyzing the energy of a sample-transmitted electron beam and displaying, in synchronism with the scan, the intensity of an electron beam that has lost energy in the sample, or the intensity ratio of electron beams that have lost different energies.
A variety of means are known for detecting the two-dimensional position information of the electron beam, such as, for example, photosensitive films, TV cameras, SSCCD cameras, and imaging plates. These record the distribution of electron beam intensity as images. Examples of devices for detecting only the position of electron beam at high speed include two-dimensional position-sensitive detectors (PSD), micro channel plates (MCP), and position-sensitive photomultiplier tubes (PSPMT).
When measuring the stress/strain of a material, it is important to measure average quantities in a specific region. Furthermore, with the recent reduction in size of semiconductor elements, it is becoming increasingly necessary to measure local quantities at high spatial resolutions on the order of several nanometers. In addition to the measurement results concerning a specific location, there is also a growing need to obtain information about the spatial distribution of stress/strain. Of course, it is expected that two-dimensional mapping would take more measurement time than a measurement of a single point and would entail the problem of ensuring positional matching between the measurement result and the measurement location.
Hereafter, it will be analyzed whether or not the conventional measuring methods satisfy the required conditions for carrying out two-dimensional mapping of the stress/strain of a material. The conditions relate to spatial resolution, measurement time, degree of high-definition, measurement precision or sensitivity, and the positional matching between measurement results and measurement location.
Both mechanical and electric resistance-wire types of strain gauges are inexpensive, but their spatial resolution is on the millimeter order at most and are therefore only capable of providing average information concerning the inside of a particular region. Further, in order to measure a different point, the strain gauge must be re-positioned, which makes it difficult to carry out two-dimensional mapping of stress/strain. While the surface acoustic wave method is capable of measuring amorphous materials, their spatial resolution is on the order of 3-100 μm, which, though higher than the spatial resolution of the strain gauge, is not sufficient for the measurement of local stress/strain in a material. The microscopic Raman method is capable of measuring the distribution of stress/strain by scanning the material surface with a laser beam. However, this method cannot be easily applied to the field of semiconductors, because it is basically incompatible with metals and its spatial resolution is 3 μm or less, which, though relatively high, is still insufficient.
While the CBED method has a high spatial resolution—not more than 10 nm—and a high accuracy, it is mostly used for crystal samples with a simple structure because of limited analysis software resources. The measurement accuracy of the method is not more than 0.02%, which is the highest among the conventional methods. But this method is not suitable for mapping because it takes time to carry out an analysis of a single point and its positional matching with the measurement location is low. Furthermore, as the method employs an electron beam of an extremely high-order diffraction, it is subject to the influences of inelastic scattering in the sample, so that the signal amount can drastically decrease with increasing sample thickness. Thus, in order to obtain a satisfactory CBED pattern using a conventional electron microscope with an acceleration voltage of 200 to 300 kV, the sample thickness must be on the order of several tens of nanometers. A resultant problem, however, is that with the decrease in the thickness of the sample, the internal stress of the sample decreases to as much as one-hundredth or so of the original stress in the case of silicon, for example. Accordingly, there is the problem that the cause of the stress affecting the semiconductor device cannot be observed in the original form.
The Fourier transform method has spatial resolution of not more than 10 nm. It is effective in the two-dimensional measurement (mapping) because it processes FFT of small divided regions using a computer. Thanks to the improved computer performance, a high mapping-result definition and a high analysis position matching can be obtained. However, the analysis accuracy drops to about 5% if a spatial resolution of 10 nm or less is sought. The method also lacks real-time capability due to the many processes implemented after the taking of an electron microscopic picture.
The nano diffraction method achieves an improved resolution by focusing the irradiation electron beam and has spatial resolution of not more than 10 nm. Its measurement accuracy is not more than about 0.1%, which is second highest, after the CBED method. The nanodiffraction method is capable of two-dimensional mapping by electron beam scanning, but lacks real-time capability because, currently, it measures the position of a specific spot in an electron diffraction image taken by a TV camera by means of image processing techniques. A higher measurement position matching can be obtained by carrying out the nano diffraction method in an apparatus supportive of the positional comparison with a scan image, such as a STEM or SEM.
It is an object of the invention to solve the problems of the prior art and provide a method and apparatus for measuring the two-dimensional distribution of stress/strain at high resolution and sensitivity, with a high level of measurement position matching, and in a real-time manner.